1. Field of the Invention
In a liquid scintillation counter, unknown radioactive samples of material which spontaneously emit electrons, are individually introduced into a sample vial or other sample receptacle in which the sample is held in a scintillator solution consisting of a solvent and a luminescent substance. The unknown material may be a spontaneous beta emission substance. The beta radiation of a radioactive isotope contained in the sample, supplies excitation energy for the solvent and luminescent substance, the beta radiation being absorbed therein. The luminescent substance when absorbing beta radiation undergoes a transition from its excited state to its ground state, emitting photons. Outside of the transparent sample vial or the like, at least one measuring transducer is provided to transform the light energy of the photons into an electrical signal. Normally the measuring transducers used are photomultipliers. In this connection it is assumed that the luminescent substance fluoresces at an energy the wavelength of which is tuned to the sensitivity of the photocathode of the photomultiplier. The higher the energy of the emitting isotope, the greater the number of molecules of luminescent substance that are excited and the greater the light intensity of the light flash caused by the respective excitation of the luminescent substance or the scintillation. The electrical signal generated by the measuring transducer normally is proportional to the light intensity and thus is proportional to the energy of the nuclear radiation which causes the excitation. Consequently, in order to discriminate between unknown sample emitters of different energy, the electrical signal generated by the measuring transducer is discriminated according to pulse height. The different pulse height ranges thus may be allocated to different energy stages of beta radiation emitting nuclides which may be contained in the respective sample of unknown material.
It is customary to suppress the noise of the photomultipliers of the measuring transducers, of which at least two are provided, by means of coincidence counting circuitry, evaluating only such events as are observed simultaneously in several measuring transducers. Furthermore, it is customary to provide at least two measuring transducers which observe the sample from diametrically opposed positions so as to obtain optimum approximation of a 4.pi. geometry.
However, when attempting to measure nuclear radiation events, it is understood that losses occur which may be classified basically into controllable and uncontrollable losses. Controllable losses are those the specific parameters of which may be kept reasonably constant during the entire measuring process. One example among a number of possibilities is the loss due to the geometric properties of the detector. As compared to controllable losses, practically the only loss which is uncontrollable is the loss due to "quenching."
Quenching losses are classified as chemical quenching and color quenching. Quenching losses occur when the energy transfer of the nuclear radiation of the sample fails to cause the luminescent substance to emit a photon. If a disturbance occurs in the energy transfer from the unknown radiating isotope sample to the solvent and from the solvent to the luminescent, the loss is called chemical quenching. Chemical quenching occurs when a molecule of the quenching substance of the scintillator solution, which molecule may originate from impurities in the sample containing radioactive traces, absorbs the energy of the solvent or luminescent substance molecule excited by the beta radiation and passes over into the ground state without emitting any radiation, the excitation energy being transformed into heat instead of a light flash.
On the other hand color quenching occurs where photons already emitted by the luminescent substance are again captured by color substances or dyes contained in the scintillator solution and reabsorbed in the wave length range of the scintillator emission so that the respective light flash is extinguished while still in the sample.
In practice there are mixed cases of chemical and color quenching which cannot be foreseen either qualitatively or quantitatively. Further complicating the problem of determining quenching is that each new sample to be measured may considerably change the specific impurities which it contains. Chemical and color quenching reduce the light emission from the scintillator solution in an unforseeable manner, not only as regards the pulse height measured, but also with respect to the number of pulses measured. The resulting pulse height spectrum is displaced toward lower pulse heights. At the same degree of quenching, a more precise pulse height spectrum is obtained with a "purely" chemically quenched sample than with a "purely" color quenched sample.
The counting efficiency ZA refers to the ratio between the events actually counted per minute (counts per minute - cpm) according to pulse height discrimination and the real decay or disintegration events per minute (disintegrations per minute - dpm): ##EQU1##
The quench corrected counting efficiency ZA referred to below is understood to be the counting efficiency corrected for the losses in the solution of a liquid scintillation system, including quenching.